Method for producing efficienty a high toughness and high tensile strength steel materials

ABSTRACT

A method for producing efficiently a high toughness and high tensile strength steel material which comprises heating a lowcarbon or low-carbon low-alloy steel slab and hot rolling the slab, said hot rolling step comprising giving the slab at least one reduction with reduction amount and temperature to be determined by N square root Ro/H in which Ro is the roll radius(mm) of the rolling mill, N is the roll rotation (rpm) and H is the slab thickness before the reduction.

United States Patent Sekine et al.

METHOD FOR PRODUCING EFFICIENTY A HIGH TOUGHNESS AND HIGH TENSILE STRENGTH STEEL MATERIALS Inventors: Hiroshi Sekine; Tadakatsu Maruyama, both of Tokyo; Katsutoshi Yamada, Tokai, all of Japan Assignee: Nippon Steel Corporation, Tokyo.

Japan Filed: Oct. l5, 1973 Appl. No.: 406,283

Foreign Application Priority Data 1 Nov. 11, 1975 [56] References Cited UNITED STATES PATENTS 3.3282ll 6/1967 Nakamura e. 148/12 F 3.645.8Ul 2/1972 Melloy et al. lllllllll l-ll/ll F 3.726.723 4/1973 Coldren et al l-l8/l2 F Primary Examiner-W. Stallard Armrney. Agent, or Firm-Toren. McGead) and Stanger {571 ABSTRACT A method for producing efficiently a high toughness and high tensile strength steel material which comprises heating a low-carbon or low-carbon low-alloy steel slab and hot rolling the slab. said hot rolling step comprising giving the slab at least one reduction with reduction amount and temperature to be determined by N VR /H in which R is the roll radius(mm) of the rolling mill. N is the roll rotation (rpm) and H is the Oct. 19. l972 Japan .l 47403991 US. Cl. 148/12 F I t. Cl. 7 F i eld of Search t?! 15 slab thckness before the reducnon 10 Claims, 3 Drawing Figures Reduction (1.)

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oow owm QQQ 0N can con o m 09 0Q cm 0 mdE METHOD FOR PRODUCING EFFICIENTY A HIGH TOUGHNESS AND HIGH TENSILE STRENGTH STEEL MATERIALS The present invention relates to a method for producing steel materials through hot rolling including steel materials which have a thickness of more than 6mm and are used as rolled, and more particularly relates to efficient production of high toughness and high tensile strength steel materials or their intermediate products using Si-Mn steel or Nb-containing steel materials under specific rolling conditions.

In case of Nb-containing steels, it has been conventionally possible to obtain high strength easily, but various proposals have been made for restricting the rolling conditions in order to assure high toughness. For example, according to a Japanese patent publication Sho 40-28241 the slab heating temperature is restricted to a temperature not higher than ll90C. and the final reduction is conducted at a temperature not higher than l000C. According to a Japanese patent publication Sho 44-27139, the slab is heated to a temperature not lower than 1230C and then given a total reduction of more than 50% of the initial thickness of the slab at a temperature not lower than 1 100C, and in the successive rolling, the steel is further given in a temperature range of 850 to 700C a total reduction of not less than 30% of the plate thickness at 850C to finish the rolling. Further according to Japanese patent publication Sho 47-506, the total reduction at a temperature not higher than lOlC is increased and the finishing rolling is conducted in a temperature range of 900 to 780C and each of the last five reductions is restricted to 5% or more.

The above conventional rolling methods have defects that production efficiency is remarkably lowered and considerable product rejects are caused due to shape defects, because the slab heating temperature must be lowered than ordinarily, or the reduction amount in the low temperature range must be increased, or the rolling must be continued down to low temperatures. Particularly, in order to assure the total reduction amount or the finish of the rolling in the low temperature zone, it is necessary to stop the rolling operation to wait for the temperature lowering of the slab. It is a very important problem to decide the temperature range, or the plate thickness at which the rolling is stopped, but this is very difficult and complicated because it depends on mill designs, slab thickness and product thickness in each of rolling shops as reported in Iron and Steel Institute Special Report", page I04, Vol. 110/122. This means that success in one mill shop or in one product can not be assured for other shops or products, and actually a plant experiment of considerably large scale has been required for each of the shop or product.

The present invention has established a hot rolling method which can solve the above defects or remarkably alleviate them and which can produce not only high tensile strength steels which can be used as rolled but also any high tensile strength steel obtained through rolling operation.

The features of the present invention are:

A. A method for producing efficiently a high toughness and high tensile strength steel which comprises subjecting a low-carbon steel slab or a low-carbon low alloy steel slab to hot rolling comprising heating the slab and giving the slab at least one reduction at a defi- 2 nite reduction temperature and reduction corresponding to N min which R is the radius of the rolling mill, N is the rotating speed (rpm) of the roll and H is the slab thickness (mm) before the reduction.

B. A method for producing efficiently a high toughness and high tensile strength steel which comprises hot rolling a low-carbon steel slab or a low-carbon low alloy steel slab, in which hot rolling the first reduction is started at a temperature of 1 C and lower and at least one reduction under the condition defined by N VR /H as in (A) above is given to the slab.

According to the present invention, the following steps are preferably added in single or in combination to the above steps;

1. 1n the case of Nb-containing steel materials, the rolling is done in such a manner that the total reduction from 950C and below to the finish of the rolling is not less than 45% of the slab thickness at 950C, and the rolling is finished in a temperature range from 880C to Ar point of the steel material.

2. In case of Si-Mn steel materials, the rolling is effected so that the total reduction from 820C to the rolling completion is not less than 20% of the slab thickness at 820C, and the rolling is finished at a temperature range from 800C to the Ar;, point of the steel material.

3. In (1) or (2), rapid cooling is conducted after the finish of the rolling with an average cooling rate between 8C/sec. and 40C/sec. and the strip is coiled at a temperature between 680 and 550C.

Reasons for limitations of the rolling conditions and the steel compositions in the present invention will be explained in detail hereinunder.

It is known that an appropriate addition of Nb in a low carbon steel will retard recrystallization of austenite after the hot rolling, and in case of Nb-containing steel which have been rolled fully at a low temperature, the transformation from the non-recrystallized austenite elongated in the rolling direction to ferrite is caused, and the nuclei of the ferrite grains generating at the time of the transformation are considerably greater in their number than when the isotropic austenite grains transform into the ferrite, so that the ferrite grains in the Nb-containing steels which have been rolled at the low temperature are finer and the strength and toughness of the steel are improved, and it is also known that the above phenomenon appears in Si-Mn steels containing no niobium when the rolling temperature is lowcred.

The present inventors have conducted extensive studies on the metallurgical processes during the hot rolling of the Nb-containing steels and Si-Mn steels and during the subsequent cooling thereof and have found that although it is very important for improvement of toughness of the Nb'containing steels and Si-Mn steels to finely recrystallize the austenite beforehand by the high temperature rolling, as disclosed in Japanese patent publication Sho 44-27139, this process can proceed at a temperature about 200C lower than the lower limit temperature disclosed in the above prior art under certain conditions in case of Nb-containing steels for example, and it has been found that the basic condition of the rolling for refining the recrystallized austenite grains is not the total reduction down to the above lower limit temperature disclosed in the prior art, but it is to maintain each reduction higher than a certain definite value determined by various factors.

It has been further discovered that the recrystallization of, even during the reduction, is suddenly completed when a large reduction larger than another critical reduction which and is generally higher than the above limit reduction. In other words, this type of recrystallization should be called as a dynamic recrystallization and the dynamically recrystallized austenite grain size is directly determined as the function only of the reduction temperature and the average strain rate during the reduction.

It has been also found that the various factors which restrict the recrystallization behavior include the strain rate of the reduction which has never been considered in the conventional rolling art, in addition to the above reduction temperature and the grain size of austenite before the reduction.

If the recrystallization is called as static recrystallization in case when the recrystallization of austenite is completed a certain time after the reduction, the austenite grain size obtained by the dynamic recrystallization is always finer than that obtained by the static re crystallization when reduced under the same combination of the reduction temperature and the strain rate. This has been also discovered by the present inventors.

In a rolling process it is known that the mean strain rate of a reduction, G, is given by the following equation:

wherein r is the amount of reduction. The strain rate, which plays an important role in controlling the recrystallization behavior of the austenite is well defined only by the parameter N VR /H.

The reason for the limitation of the lower limit of each reduction in the rolling within the specific temperature zone in the present invention is that the present invention aims to positively refine the austenite grains through the dynamic recrystallization or at least through a rapid static recrystallization by each reduction in the above specific temperature range according to the above discovery.

The limitation of the temperature range of 1 150 to 980 C for Nb-containing steels and 1 150 to 850C for Si-Mn steels in which the specific reduction is conducted is based on the discovery that the dynamic recrystallization is caused relatively easily even by the reduction schedule hitherto practised in rolling mills if the temperature is higher than 1 150C and a certain refinement of the austenite is attained, and that in case of the Nb-containing steels to which the present invention is applied, the dynamic recrystallization can proceed even by the reduction at 980C at least if certain condition is satisfied, and similarly by the reduction at 850C in case of Si-Mn steels.

The reason for defining the case in which the reduction is completely eliminated at temperatures above ll50C is that in the case when high efficiency as a whole including steps prior to the hot rolling is obtained if the hot rolling is done with a relatively thin steel slab or in case when the product thickness is large, or in case when it is necessary to maintain the total reduction at the low temperature zone as defined in the additional feature (1) or (2) of the present invention for obtaining very high toughness, it is necessary to re- 4 duce the total reduction number in order to maintain each reduction between 1l50 and 980C or ll50C and 850C not smaller than the lower limit, and in such a case it is desirable for assuring the toughness to eliminate completely the reduction above ll50C.

FIG. 1 and FIG. 3 show the results of extensive rolling experiments seeking for the lower limit reduction for the dynamic recrystallization of the austenite by each reduction between 1 150C and 980C or between 1 150C and 850C in relation to the reduction temperature, the rotating speed of the roll, the roll diameter and the plate thickness before the reduction, and these experiments were conducted on assumption of an ordinary process in which the Nb-containing steel or the Si-Mn steel is given continuous reductions and, the austenite grains are gradually refined.

FIG. 2 shows results of experiments for determining the lower limit reduction necessary for the dynamic recrystallization of austenite grains to be caused when the first reduction in the temperature range between l150 and 1050C is given to the Nb-containing steel which has been heated at high temperatures as usually and particularly about 1200C at which the austenite grains remarkably coarsen and cooled without any reduction down to ll50C at all. The lower limit reduction is shown as the function of the various rolling conditions as in FIG. 1. Reasons for the definition that at least one reduction larger than those defined in FIG. 1 and FIG. 2 is given are that when only one reduction most disadvantageous to the refinement of the austenite is given under the condition using a strong rolling mill, recrystallized grains of more than No. 5 of the austenite grain size are obtained and if the production process is conducted thereafter in combination with the desirable steps of l), (2) and (3) a satisfactory fine structure of transformation product can be obtained and satisfactorily high strength and toughness are assured for the final products.

In the present invention, Nb-containing steels and Si-Mn steels are described separately because the presence of Nb changes considerably the lower limit reduction for the dynamic recrystallization and the reason for the limitation of the carbon content to 0.30% or less in the steel composition applicable to the present invention is that more than 0.30% carbon remarkably deteriorates weldability of the steel and thus undesirable for the steel which requires rolling operations. Reason for the limitation of the niobium content to 0.15% or less in the steel composition in Nb-containing steels is that if the niobium content is more than 0.15%, it is impossible to prevent the raising the lower limit of the temperature at which the austenite can be recrystallized during the rolling, and thus the conditions defined in FIG. I are changed even when the carbon and niobium contents are considerably reduced. Further, the alloying elements other than carbon and niobium are not specifically limited in the present invention because it has been confirmed through experiments that the above facts are not changed quantitatively when Si, Mn, Ni, Cr, Cu and V are varied substantially.

In case of Si-Mn steels containing no niobium, silicon and manganese are dispensable for increasing the strength at low cost, but silicon contents more than 0.70% coarsen the ferrite grains, and manganese contents more than 2.0% cause problems in the steel making process and in connection with weldability. Thus the upper limits of these elements are defined as above.

Further, not only the steel materials which are used as rolled and steel materials of ferrite-pearlite structure, but also any high-tensile steel material to be obtained through hot rolling is included in the scope of the present invention. This is based on the following known facts and newly discovered facts. In case when the structure after cooling is mainly of bainite, even when the steel is quenched immediately after or a certain time after the rolling and then tempered, the finer the austenite before the transformation is, the better the toughness of the product is. Also in case when the steel is left to cool or is cooled at controlled cooling rate after the rolling, reheated, quenched and tempered or normalized, the finer the structure after the hot rolling and the cooling is, the more improved the toughness of the product is.

The present invention is based on the completely new discoveries including the results of the quantitative experiments and has been established as a controlled rolling method in which the rolling pass schedule in the high temperature range is controlled, and at least one large reduction is given in the high temperature range. This is completely different from the conventional controlled rolling method in which the rolling pass schedule in the low temperature is controlled.

As the result, according to the present invention, if the design is made in such a manner that the austenite recrystallization by the rolling at the specific high temperature range is occupied mainly by the dynamic re crystallization, the considerations to the temperature and the time as disclosed in the Iron and Steel Institute Special Report" for waiting for the slab temperature lowering by stopping the rolling operation in the specific temperature range are not required.

Further according to the present invention, the rolling at the high temperatures where the steel material is still soft enough is controlled so as to refine the austenite and to assure the same'level of toughness, while the rolling at low temperature where the steel material becomes hard is alleviated, the waiting time for the slab temperature lowering for the low temperature is saved, thus increasing the production efficiency, reducing the defects such as plate waviness and crowns due to the low temperature rolling, improving the production yield, and making possible to effect the rolling by a relatively light rolling mill.

Further the present invention has advantages that only by incorporating the effects of strain rate into the rolling schedule it is no more necessary to translate with great efforts a successful rolling operation at a specific rolling shop or with a specific plate thickness into a rolling operation of different specification, and it is possible to produce good rolled products only with a minimum plant experiments at a rolling shop having no experience of controlled rolling. The above advantages of the present invention have great industrial significance.

As above mentioned, it is one of the features of the present invention to select the rolling conditions so as to make the austenite take the dynamic recrystallization between l 150 and 980C or l 150 and 850C, but it shold be understood that the present invention also include the cases in which the recrystallization of the austenite in the above temperature range is not the dynamic recrystallization, that is, the recrystallization is completed a certain time after the reduction.

The reasons are as follows: the curves in FIGS. 1 and 3, or the curves in FIG. 2 have been respectively sought for on the basis of the austenite grain size when the ordinary reduction schedule is given or the austenite grain size before the first reduction when the average slab heating is effected. Therefore, it is probable that the dynamic recrystallization does not take place but instead the static recrystallization takes place depending on the slab heating or the rolling history up to the reduction specified by the present invention even when the rolling conditions of FIGS. 1 to 3 are satisfied. Even in this case, if each of the reductions is made so large, the completion of the recrystallization is promoted and the recrystallized grains are refined, and thus most of the objects of the present invention can be attained.

in the present invention, the following alloying elements may be added in addition to C, Mn, and Nb for various purposes in combination with the additional production conditions of l) to (3).

According to the present invention, the reductions in the high temperature range of l to 980C or 1 150 to 850C are specified, but when the low temperature rollings as specified in (l) and (2) are applied in combination, the strength and toughness can be further improved.

The above technical thought has some connection with the rolling type in which the rolling pass schedule in the low temperature range is controlled, but the temperature of 950C or lower for Nb-containing steel or of 820C or lower for Si-Mn steel defined for the total reduction and the temperature of 880C or lower for Nb-containing steel or of 800C or lower for Si-Mn steel defined for the hot rolling completion are high as compared with the conventional arts. Namely, in the present invention, the restrictions in the low temperature range are alleviated as compared with the conventional art for the production of the same steel grades by specifying the high temperature rolling conditions, thus improving the production efficiency and yield.

The cooling and the coiling conditions after the rolling as defined in the preferably condition (3) are directed to the case when the structure after the coiling is mainly of ferrite-pearlite. It is known that the intermingling of the bainite into the structure after the transformation is harmful] for the toughness of the steel material, and it has been found that it is effective for the refinement of the ferrite to effect a rapid cooling at the time of the transformation so far as the bainite is not present.

In a continuous rolling, when the cooling after the rolling is made rapid and the coiling temperature is lowered, the ferrite becomes finer, but the bainite appears during the cooling or after the coiling under a certain condition. This limit cooling rate is 40C/sec. and the limit coiling temperature is 550C.

The cooling rate of 8C/sec. corresponds to the considerable slow cooling side of the conventional continuous hot rolling, and at such a cooling rate the coiling will be naturally a high temperature coiling. The low temperature coiling enhances the effectiveness of precipitation hardening by Nb, and V. Therefore, with a cooling rate as slow as 8C/sec. the strength is naturally low but excellent toughness is assured.

lt is preferable to add 0.01 to 0.70% Si and 0.70 to 2.00% Mn to the Nb-containing steel too, from the necessity of the steel making and for assuring the strength at low cost. Lowering of the carbon is also preferable for assuring the toughness and the weldability, but in case of ferrite-pearlite steels in particular, the increase of the transformation temperature thereby is not desir- 7 able because it reduces the precipitation hardening by Nb, and V, coarsens the ferrite grains and deteriorates the toughness. Therefore, in the case of Nb-containing steels it is preferable to define (C Mn Nb) Niobium, when added with the other elements, is effective to improve the toughness and strength, but should be maintained lower from the point of weldability. It also changes partially the conditions shown in FIG. I through its precipitation of NbN into the austenite. Therefore it is preferable to maintain the nitrogen content not more than 0.010%.

Further, not more than 0.08% A1 may be added from the necessity of the steel making, not more than 0.20% V may be added for obtaining desirable strength, not more than 0.005% B may be added for assuring hardenability as required for tempered steels, not more than 1.50% Ni may be added for assuring the toughness. strength and the transformation temperature, not more The Nb-containing killed steels A and B which were prepared in a convertor were made into flat slabs of 240mm thickness, heated 1300C, rolled to 82mm thickness by three reductions down to 1150C, subjected to the reduction at three temperatures in combination as shown in Table 3 at a constant reduction as shown in Table 2 using the roll groups specified in Table 2 for the fourth, fifth and sixth reductions, and then further rolled into 9mm thickness by twelve reductions in total, and coiled at 650C. The rolling schedule 1 in Table 3 represents an ordinary rolling with no restriction, and the rolling schedules 2 and 3 in which the temperature was restricted had longer rolling times than the ordinary rolling schedule l by 50 and 90 seconds respectively. Table 3 shows also the values of N V R lH and the lower limit reduction values determined from FIG. 1 for the dynamic recrystallization sought for from the reduction temperatures shown in Table 3.

than 2.00% Cu may be added for the corrosion resis- Table 2 tance and the strength, not more than 1.00% Cr may be added for the strength and the transformation temperaspefiificationpf Rollins Mill and Reduction Allotment ture, not more than 0.20% Ce or misch metal may be Fourth Fifth Sixth added for controlling the shape of sulfides and improv- Reduction Reduction Reduction ing the toughness and workability in the direction per- R0" Radius pendicular to the rolling direction, and not more than (m 57 5 365 (0.15 Nb%) Ti only for Nb-containing steel or not 222:? Speed more than 0.10% Zr may be added for adjusting the Change in Slab shape of sulfides, absorbing N, and restricting the 82/48 48/3 31/245 coarsening of the austenite grains during the reheating. 11,321,221 413 35,1 2| The above additions do not substantially change the N f 1"" 183 147 limit reduction defined in FIGS. 1 and 3. The reason for limiting the titanium content in connection with the ni- Table 3: Rolling Temperatures and Lower Limit Reductions for Dynamic Recrystallization Fourth Fifth Sixth Final Reduction Reduction Reduction Reduction Rolling Temperbower Temper- Lower Temper- Lower Temper- Schedule ature Limit ature Limit ature Limit ature (C) Reduc- (C) Reduc- ("C) Reduc- PC) tion tion tion (%J (M obium content is that the titanium addition also contributes to restrict the austenite recrystallization during the hot rolling similarly as the niobium content.

The present invention will be more clear from the fol- From the comparison of Tables 2 and 3, the followlowing examples.

EXAMPLE 1 Table 1 shows the chemical compositions of the steel materials used in present invention.

Table 1 Chemical Compositions of Steel Materials (in weight ings are clear. In Table 3, the rolling schedule 1 satisfies the present invention in its fourth and fifth reductions, but includes the sixth reduction between 1150 and 980C which does not satisfy the present invention and thus is outside the scope of the present invention. Whereas in the rolling shedules 2 and 3, not only the fourth and fifih reductions satisfy the present invention, but also the sixth reduction is effected at temperatures not higher than 980C and thus no restriction for the reduction is required. Namely the rolling schedules 2 and 3 are within the scope of the present invention. Mechanical properties at the coil center portion of the steel strip thus obtained are shown in Table 4.

Table 4 Mechanical Properties of Rolled Steels (A and B) Sample Rolling Yield Tensile Elonga- L vTrs dwtt Trs Steels Schedule Point Strength tion (Kglmm (Kg/mm) g-m) l A 1 37/40 49/49 39/3l l.2/2.8 3()/5O 2l/42 2 38/42 50/52 38/32 2.5/2.9 -52/68 4l/54 3 43/47 54/56 36/29 1.7/2.6 44/74 23/42 B 1 37/40 H52 38/32 3.2/3.6 52/-74 -43/ 4s 2 40/45 54/56 38/29 2.7/3.0 67/-74 -53/62 3 42/46 55/57 37/28 2.6/3.0 -63/78 49/55 suhsize Tensile Properties: Rolling Direction/Perpendicular to Rolling Direction Toughness. Worst/Best it is understood from the comparison of the rolling conditions shown in Table 3 and the mechanical properties shown in Table 4 that both of the strength and the toughness of the steels A and B obtained by the rolling schedules 2 and 3 are better than those obtained by the rolling schedule 1. It is also understood from the comparison of the schedules 2 and 3 that the schedule 3 in which the waiting time of 40 seconds in the course of the rolling for assuring the low temperature rolling shows somewhat higher strength for both of the steels A and B than the schedule 2, but shows no tangible improvement in the toughness. Whereas the schedule 2 gives very stable toughness as compared with the schedules 1 and 3. The schedule 2 is the typical embodiment of the present invention which can produce a high toughness and high tensile strength steel at a high production efficiency without applying a very low temperature rolling.

EXAMPLE 2 The killed steel C containing Nb and V, prepared in a. convertor and having the chemical composition shown in Table l was made into a flat slab of 215mm thickness, heated to l300C, and rolled to. 76mm thickness by three reductions down to 1 150C, subjected to three rolling schedules with the combination of the reduction temperatures and the reductions shown in Table 5, using the roll groups shown in Table 2 for the fourth, fifth and sixth reductions, further rolled to 12.7mm thickness by eleven reductions in total, and coiled at the temperatures shown in Table 5.

The rolling schedule 4 in Table 5 represents an ordinary rolling with no restriction. The schedule 5 had a longer rolling time than the ordinary rolling schedule by 60 seconds, and in the rolling schedule 6, waiting time was of 70 seconds provided on the waiting side table between the fourth and the fifth reductions.

The figures in the parenthesis in Table 5 represent the lower limit reduction amount for the dynamic recrystallization for each reduction sou ht for from FIG. 1 on the basis of the value of N R /H and the reduction temperatures in the table. The rolling schedule 4 in Table 5 includes the sixth reduction which does not reach the lower limit reduction amount determined from FIG. 1 between ll and 980C, while in the rolling schedules 5 and 6, the fourth and the fifth reductions are over the reduction amounts given by FIG. 1 and yet the sixth reduction is 970C in the schedule 5 and 965C in the schedule 6, both being outside the restricted temperature range. Thus the schedules 5 and 6 are within the scope of the present invention.

The mechanical properties of the steel strips thus rolled are shown in Table 6, from which it is clear that the schedules 5 and 6 give excellent strength and toughness in spite of the thickness of 12.7mm as compared with the schedule 4. Remarkable amount of bainite was observed in some of the steel materials rolled by the schedule 4 in spite of the high temperature coiling, while in the steel materials rolled by the schedules 5 and 6, no such structure was observed. The extension of rolling time of seconds in the schedule 5 causes only about 30% lowering in the production efficiency in the continuous hot rolling mill where the present invention was applied. In schedule 6, the lowering of the production efficiency was completely eliminated by the use of the side table. Thus it is understood that the present invention represented by the schedules 5 and 6 assures remarkable efiiciency for the production of steel plates of thickness and quality as above.

Table 5 Rolling Schedules and Lower Limit Reductions for Dynamic Recrystallization Tensile Properties: Rolling Direction/Perpendicular to Rolling Direction: Toughness: Worst/Best EXAMPLE 3 Steel C containing niobium and vanadium shown in Table l made into flat slabs of 100mm thickness, heated to l250C, cooled without any reduction down to ll50C and three reductions were made between [150 and 980C according to the rolling schedule shown in Table 7, using the roll groups shown in Table 7 for the first and the third reductions, and subsequent rolling was made to obtain steel plates of 12.9mm thickness with the total nine reductions and the plates were coiled at 670C. The figures in the parenthesis in Table 7 the lower limit reduction amounts for the dynamic recrystallization for each reduction, determined from FIG. 2 for the second reduction, from FIG. 1 for the first and third reductions on the basis of the values of N R /H and the reduction temperatures. From Table 7 it is understood that the rolling schedule 7 satisfies the rolling conditions of the present invention.

Table 7 in Table 9, using the rpm-variable rolls of 500mm radius to obtain steel plates of 19mm thickness.

Table 8 Chemical Composition of Steel D C Si Mn 8 Nb V Specification of Rolling Mill, Rolling Schedule and Lower Limit Reductions for Dynamic Recrystallization The mechanical properties near the coil head portion of thus rolled steel plates are shown at the lowest column in Table 6. It is understood that the schedule 7 which is within the scope of the present invention gives better material properites that the schedule 5 in spite of the thickness of 12.9mm. This example represents a case where the thickness of slabs introduced to a rolling mill is relatively thin. Even in such a case, a fine structure which gives desired excellent strength and toughness can be obtained by completely eliminating reductions above 1 150C and by increasing enough the amount of the first reduction at temperatures lower than llC. Almost no intermingling of bainite was observed in the rolled structure of the above material.

EXAMPLE 4 The killed steel D shown in Table 8 containing Nb and V, which was prepared in a convertor was made into flat slabs of 220mm thickness, heated to l250C, and rolled according to the two rolling schedules shown thus outside the scope of the present invention, but the lower temperature reductions were made at far lower temperatures and the total reduction in the low temperature zone is greater as compared with the schedule 8.

Mechanical testings were done on the top, middle and bottom portions of the steel plates rolled and the measurements as average of the three portions are shown in Table 10.

From the comparison of the mechanical properties shown in Table I0 and the rolling conditions shown in Table 9 it is understood that the schedule 8 which is within the scope of the present invention gives better strength and toughness than the schedule 9 which is outside the scope of the present invention. In case of the steel plates which were rolled by the schedule 8, despite the thickness of 19mm and the fact that the lower temperature zone reductions which remarkably lower production yield and efficiency were not strengthened particularly, excellent strength and toughness due to a uniform structure of very fine grains of ferrite grain 3 ,9 l 8 ,999 13 14 number of about 12.5 are obtained. This means that the amount for the dynamic recrystallization, and the subpresent invention is useful for production of such thick sequent reductions were started from 800C, with the steel plates. total reduction of 25% between 800 and 750C. Thus Table 9 Rotating Speed of Roll (rpm) 29 21 44 63 to l9mm thickness Changes in Slab l66/l 33 133/97 97]63 7 passes (MO/780C) Thickness (mm/mm) Finishing Temperature 780C Reduction (7%) l9.9 27.1 35.l Total Reduction in Low Temperature Zone Reduction Temperl l l080 l0lOC ature (C) Lower Limit 24 23 33.5

Reduction (70) Schedule Sixth Seventh Eighth Low Temperature Rolling 9 Reduction Reduction Reduction Condition Rotating Speed of Roll (rmp) 2B 3] 3! 79 to 19mm thickness Changes in Slab l21/l05 l05/9l 9l/79 8 passes ISM/730C) Thickness (mm/mm) Finishing Temperature 730C Reduction (75) I32 13.3 13.2 Total Reduction in Low Temperature Zone N VR /H (rpm) 57 68 72 76% Reduction Temper- 1010 1000 990 ature (C) Lower Limit 29 33 37 Reduction (76) Table 10 Mechanical Properties of Steel D of l9mm Thickness Measured at the middle portion of the steel plate. Tensile Properties: Rolling Direction/perpendicular to Rolling Direction: Toughness: Worst/Best 4 the schedule l0 is within the scope of the present in- EXAMPLE 5 0 vention. Whereas in the schedule ll, low reductions The killed Si-Mn steel E prepared in a convertor and below the lower limit reduction for the dynamic recryshaving the chemical composition shown in Table 11 tallization were conducted at temperatures not lower was made into flat slabs of 245mm thickness, heated to than 1000C and thereafter the reductions were made l250C, and rolled according to the two rolling schedaccording to the same schedule 10. Thus the schedule ules shown in Table 12 using the rolls of 485mm radius ll is outside the scope of the present invention. of fixed rotation rate of 40 r.p.m. to obtain steel plates Mechanical properties of the Si-Mn steel plates of of 30mm thickness. 30mm thickness as rolled above are shown in Table 13 Table 1 l as average of the four portions from the top to the bottom.

C Chersrpcal Com'pornsition of Stgel E 5 When the mechanical properties shown in Table 13 and the rolling conditions shown in Table 12 are com- L36 (1013 0-005 pared, it is understood that the schedule 10 which is within the scope of the present invention gives better strength and toughness than the schedule ll which is Table 12 shows the values of N P H the i outside the scope of the present invention. The results temperature the lower ,hmlt redufnon obtained by the schedule 10 clearly indicates that the amounts for dynam'c recrysmlhzauon for the 'M present invention is useful for production of the Si-Mn Steel determined from 1n the sqhefiule P steel plates as thick as of 30mm thickness and not contenth reduction was over the lower limit reduction mining alloying elements Such as niobium Table I2.

Finishing Finishing Rolling Rolling Low Temperature Ninth Tenth Rolling Reduction Reduction Condition Schedule l0 Rotating Speed of Roll 40 40 Table 12.-continued Finishing Finishing Rolling Rolling Low Temperature Ninth Tenth Rolling Reduction Reduction Condition (rpm) for schedules 9 and 10 Changes in Slab Thickness(mm/mm) 70/55 55/40 40 to 30mm thickness Reduction ('1 1 21.4 27.3 five reduction N \/R,,/H (rpm) 105 119 (800C750C) Total Reduction Reduction Temperin low temper ature (C) ll0 I000 ature zone 25% Lower Limit Reduction (7H 25 26 Schedule ll Rotating Speed of Roll 40 40 (rpm) Changes in Slab Thickness(mm/mm) 65/50 50/40 Reduction (2) 23.1 20.0 N R,,/H (rpm! 109 125 Reduction Temperature (C) 1000 1000 Lower Limit Reduction Wt] 26 26.5

Table 13 Mechanical Properties of Steel E Rolling Yield Tensile ElongavE vTrs Schedule Point Strength tion (Kg/mm) (Kg/mm) (kg-m) Worst! Best What is claimed is:

l. A method for efiiciently producing a high toughness and high tensile strength steel material which comprises heating a low-carbon or low-carbon, low-alloy steel slab which contains not more than 0.30% C, and not more than 0.15% Nb, and hot rolling the slab, said hot rolling step comprising giving the slab at least one reduction in an amount not smaller than that shown by each of the curves or the interpolated curves shown in FIG. 1 as determined by the constant N IRJH wherein R is the roll radius (mm) of the rolling mill, N is the roll rotation speed (rpm) and H is the slab thickness before the reduction.

2. A method for producing efficiently a high toughness and high tensile strength steel material, which comprises heating a steel slab containing not more than 0.30% C and not more than 0.15% Nb to a temperature required for assuring required strength of the steel slab, and hot rolling the slab, said hot rolling comprising giving the slab between 1 150 and 980C at least one reduction including a reduction conducted at the lowest temperature within the above temperature range with a reduction amount not smaller than that shown by each of the curves or the interpolated curves shown in FIG. 1 in accordance with the constant N V R,,/H as defined in claim I and the reduction temperature.

3. A method for producing efficiently a high toughness and high tensile strength steel material, which comprises heating a steel slab containing not more than 0.30% C, not more than 0.70% Si and not more than 2.00% Mn to a temperataure required for assuring required strength of the steel slab, and hot rolling the slab, said hot rolling comprising giving the slab between 1 and 850C at least one reduction including a reduction conducted at the lowest temperature within the above temperature range with a reduction amount not smaller than that shown by each of the curves or the interpolated curves shown in FIG. 3 in accordance with the constant N R /H as defined in claim 1 and the reduction temperature.

4. A method according to claim I in which the first reduction is started at a temperature not higher than 1 150C.

5. A method according to claim 1 in which the first reduction is started at a temperature not higher than 1 150C.

6. A method for producing efficiently a high toughness and high tensile strength steel material, which comprises heating a steel slab containing not more than 0.30% C, and not more than 0.15% Nb to a temperature required for assuring required strength and hot rolling the steel slab, said hot rolling comprising starting the first reduction at a temperature not higher than 1 150C, giving the slab during the rolling between 1 150 and 980C including the first reduction at least one reduction including the reduction conducted at the lowest temperature within the above temperature range with a reduction amount not smaller than that shown by each of the curves or the interpolated curves shown in FIG. 1 in accordance with the constant N V R /H as defined in claim I and the reduction temperature.

7. A method for producing efficiently a high toughness and high tensile strength steel, which comprises heating a steel slab containing not more than 0.30% C and not more than 0.l5% Nb to a temperature not lower than 1200C, hot rolling the slab, said hot rolling comprising starting the first reduction at a temperature not higher than 1150C with a reduction amount not smaller than that shown by each of the curves or the interpolated curves shown in FIG. 2 in accordance with the constant N R IH as defined in claim 1 and the reduction temperature, and conducting subsequent reductions down to 980C including at least one reduction including the reduction at the lowest temperature between 1 150 and 850C with a reduction amount not smaller than that shown by each of the curves or the interpolated curves in P16. 1.

8. A method for producing efficiently a high toughness and high tensile strength steel material, which comprises heating a steel slab containing not more than 0.30% C, not more than 0.70% Si, and not more than 2.00% Mn to a temperature required for assuring required strength and hot rolling the steel slab, said hot rolling comprising starting the first reduction at a temperature not higher than 1 150C, giving the slab during the rolling between ll50 and 850C at least one re- 18 duction including the reduction conducted at the lowest temperature within the temperature range with a reduction amount not smaller than that shown by each of the curves or the interpolated curves in FIG. 3 in accordance with the constant N V R /H as defined in claim 1 and the reduction temperature.

9. A method for efficiently producing a high toughness and high tensile strength steel material which comprises heating a low-carbon or low-carbon, low-alloy steel slab which contains not more than 0.30% C, not more than 0.70% Si, and not more than 2.0 Mn, and hot rolling the slab, said hot rolling step comprising giving the slab at least one reduction an an amount not smaller than that shown by each of the curves or the interpolated curves shown in FIG. 3 as determined by the constant N V R /H wherein R,, is the roll radius (mm) of the rolling mill, N is the roll rotation speed (rpm) and H is the slab thickness before the reduction.

10. A method according to claim 9 in which the first reduction is started at a temperature not higher than 1 C.

Patent No.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Page 1 of 3 Dated November L1, 1975 lnventofls) Hiroshi Sekine; 'Iadakatsu Maruyama; Katsutoshi Yamada It is certified that error appears in the above-identified patent I and that said Letters Patent are hereby corrected as shown below:

Column Column Column Column Column Column Column Line Line Line .Line

Line

Line

Line Line Line Line Line Line Line Line Line 36, change "conventional" to prior--; 52, change "page 104, Vol. 110/122 to --Vol. 104, pages ll0/l22--;

change "rolling" to --reduction--;

26, change "55" to --fi--;

31, the structural formula should appear as follows:

E N Ro/H 1 36, after "N JR insert and r--;

23, change "about" to -above--;

28/29, change "and FIG. 2" to --or FIG. 3-;

52, after "raising" insert --of--;

55/56, change "niobium" to -nitrogen-;

62, change "shold" to --should--;

63, change "include" to includes-;

3, change "average" to -conventional---;

15 Cy Si 21, change "rollings as specified in (l) and (2) are" to -rolling as specified in (l) or (2) is":

33, change "conventional"-to'--prior--;

34/35 change "conventional" to --prior;

46, I after "transformation" insert and delete "so far as the bainite is not preaeni- Patent No.

Hiroshi Sekine; Tadakatsu Maruyama; Katsutoshi Yamada UNITED STATES PATENT OFFICE Page 2 of 3 CERTIFICATE OF CORRECTION Dated November 11, 1975 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Line Line Column 7,

Line Line Line Line Line Column 8,

Line- Line Column 9, Line Line Line Line Line Line Line Line Column 11,

change "Niobium" t0 -Nitrogen;

after "required for" insert quenched and--; after change change "FIG.

"restrict" "content" 1" change "and" to --to-; to retard; to -addition-;

change "waiting time" to further waiting-: after "seconds" insert --occurs-; change "a flat slab" to flat slabs;

after "Table 1" insert -was-;

change "flat slabs" to'--a flat slab-; change "second" to --first--;

change "first" to --second-;

change "properites that" to properties as compared with";

UNITED STATES PATENT OFFICE page 3 of 3 CERTIFICATE OF CORRECTION Patent No. 3, 918,999 Dated November 11, 1975 Inventor) Hiroshi Sekine; Tadakatsu Maruyama; Katsutoshi Yamada It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 13, Table 9, Schedule 9, Line N -/R 7H (rpm) under "Eighth Reduction" change "72" to -73--;

Column 15, Table l-2-continued, below the heading "Low Temperature Rolling Conditions" change "schedules 9 and 10" to read -schedules l0 and ll--.

Signal and Scaled this Twenty-fourth Day. of August 1976 [sen] Anest:

RUTH C. MASON C. MARSHALL DANN 8 m (nmmissluner 0] Parents and Trademarks 

1. A METHOD FOR EFFECIENTLY PRODUCING A HIGH TOUGHNESS AND HIGH TENSILE STRENGH STEEL MATEIAL WHICH COMPRISES HEATING A LOW-CARBON OR LOW-CARBON, LOW-ALLOY STEEL SLAB WHICH CONTAINS NOT MORE THAN 0.30%C, AND NOT MORE THAN 0.15% NB, AND HOT ROLLING THE SLAB, SAID HOT ROLLING STEP COMPRISING GIVING THE SLAB AT LEAST ONE REDUCTION IN AN AMOUNT NOT SMALLE THAN THAT SHOWN BY EACH OF THE CURVES OR THE INTERPOLATED CURVES SHOWN IN FIG. 1 AS DETERMINED BY THE CONSTANT N $ R*/H WHEREIN R* IN THE ROLL RADIUS (MM) OF THE ROLLING MILL, N IS THE ROLL ROTATION SPEED (RPM) AND H IS THE SLAB THICKNESS BEFORE THE REDUCTION.
 2. A method for producing efficiently a high toughness and high tensile strength steel material, which comprises heating a steel slab containing not more than 0.30% C and not more than 0.15% Nb to a temperature required for assuring required strength of the steel slab, and hot rolling the slab, said hot rolling comprising giving the slab between 1150* and 980*C at least one reduction including a reduction conducted at the lowest temperature within the above temperature range with a reduction amount not smaller than that shown by each of the curves or the interpolated curves shown in FIG. 1 in accordance with the constant N Square Root Ro/H as defined in claim 1 and the reduction temperature.
 3. A method for producing efficiently a high toughness and high tensile strength steel material, which comprises heating a steel slab containing not more than 0.30% C, not more than 0.70% Si and not more than 2.00% Mn to a temperataure required for assuring required strength of the steel slab, and hot rolling the slab, said hot rolling comprising giving the slab between 1150* and 850*C at least one reduction including a reduction conducted at the lowest temperature within the above temperature range with a reduction amount not smaller than that shown by each of the curves or the interpolated curves shown in FIG. 3 in accordance with the constant N Square Root Ro/H as defined in claim 1 and the reduction temperature.
 4. A method according to claim 1 in which the first reduction is started at a temperature not higher than 1150*C.
 5. A method according to claim 1 in which the first reduction is started at a temperature not higher than 1150*C.
 6. A method for producing efficiently a high toughness and high tensile strength steel material, which comprises heating a steel slab containing not more than 0.30% C, and not more than 0.15% Nb to a temperature required for assuring required strength and hot rolling the steel slab, said hot rolling comprising starting the first reduction at a temperature not higher than 1150*C, giving the slab during the rolling between 1150* and 980*C including the first reduction at least one reduction including the reduction conducted at the lowest temperature within the above temperature range with a reduction amount not smaller than that shown by each of the curves or the interpolated curves shown in FIG. 1 in accordance with the constant N Square Root Ro/H as defined in claim 1 and the reduction temperature.
 7. A method for producing efficiently a high toughness and high tensile strength steel, which comprises heating a steel slab containing not more than 0.30% C and not more than 0.15% Nb to a temperature not lower than 1200*C, hot rolling the slab, said hot rolling comprising starting the first reduction at a temperature not higher than 1150*C with a reduction amount not smaller than that shown by each of the curves or the interpolated curves shown in FIG. 2 in accordance with the constant N Square Root Ro/H as defined in claim 1 and the reduction temperature, and conducting subsequent reductions down to 980*C including at least one reduction including the reduction at the lowest temperature between 1150 and 850*C with a reduction amount not smaller than that shown by each of the curves or the interpolated curves in FIG.
 1. 8. A method for producing efficiently a high toughness and high tensile strength steel material, which comprises heating a steel slab containing not more than 0.30% C, not more than 0.70% Si, and not more than 2.00% Mn to a temperature required for assuring required strength and hot rolling the steel slab, said hot rolling comprising starting the first reduction at a temperature not higher than 1150*C, giving the slab during the rolling between 1150* and 850*C at least one reduction including the reduction conducted at the lowest temperature within the temperature range with a reduction amount not smaller than that shown by each of the curves or the interpolated curves in FIG. 3 in accordance with the constant N Square Root Ro/H as defined in claim 1 and the reduction temperature.
 9. A methOd for efficiently producing a high toughness and high tensile strength steel material which comprises heating a low-carbon or low-carbon, low-alloy steel slab which contains not more than 0.30% C, not more than 0.70% Si, and not more than 2.0 % Mn, and hot rolling the slab, said hot rolling step comprising giving the slab at least one reduction an an amount not smaller than that shown by each of the curves or the interpolated curves shown in FIG. 3 as determined by the constant N Square Root Ro/H wherein Ro is the roll radius (mm) of the rolling mill, N is the roll rotation speed (rpm) and H is the slab thickness before the reduction.
 10. A method according to claim 9 in which the first reduction is started at a temperature not higher than 1150*C. 